The present invention relates to the co-extrusion of pluralities of flowing polymer plastic streams through nozzle extruders and the like into injection molding and similar apparatus for forming multi-layer plastic articles in which an interior core is encased by inner and outer layers of the article; and, more particularly, to the control of relative volumetric flow rates of the layers for attaining greater flexibility in the properties and relative thickness and positions of the layers in the ultimate article. More specifically, the invention is especially, though not exclusively, useful with co-extrusion processes of the type described in my earlier U.S. Pat. No. 5,914,138, issued Jun. 22, 1999 For Apparatus For Throttle-Valving Control For The Co-Extrusion Of Plastic Materials As Interior Core Streams Encased By Outer And Inner Streams For Molding And The Like.
A common problem in multilayer molding is the maintaining of a uniform penetration of the leading edge of the interior core layer, when that layer is not near the zero gradient of the velocity profile of the flowing polymer stream as it flows through a hot runner nozzle and/or into the mold cavity forming the molded article. Unlike the prior art tapered leading edge flow of, for example, systems of the type disclosed in U.S. Pat. Nos. 4,895,504 and 4,892, 699, my said earlier patent teaches the combining of the different flow streams of materials to achieve a velocity profile of the combined streams in the melt delivery system that is similar to the velocity profile of the combined stream in the injection mold cavity, thereby insuring uniformity in a resulting molded article.
This problem of maintaining uniform penetration of the leading edge of the interior core layer when it is not close to the zero gradient of the velocity profile becomes particularly severe when there is the requirement to form the multilayer article with the core layer not centered on the midplane of the article.
In two-material, three-layer preform molding, for example, it may be desirable to have a barrier or scavenger layer closer to either the inner sidewall or outer sidewall of a blow-molded container article, in order to enhance the barrier property of the container. In three-material, four-layer preform molding, this leading edge problem also occurs, particularly when the volumetric flow rate of one of the interior core layers is greater than that of the other interior core layer.
Another common current problem also arises in using post-consumer recycled plastic (PCR) in a molded article that consists of layers of two other polymers. Current art accomplishes this three-material combination by using apparatus and methods that create a 5-layer article. With such 5-layer technology, however, the molding cycle times are significantly longer than if the article had been molded of only one material. Such 5-layer molded articles, moreover, suffer delamination of the layers if the second polymer has low adhesion to the virgin skin layers and to the central PCR layer.
The present invention is directed to the solution of the above problems, and limitations, among others, in such prior art systems through a later-described technique for permitting changed or controlled variation of the relative volumetric flow rates of the inner and outer layers after the flow of the interior core layer stream has started.
A principal object of the present invention, accordingly, is to provide a new and improved method of and apparatus for molding multi-layer polymer plastic articles having inner, outer and interior or core layers that shall not be subject to such problems and limitations; but that, to the contrary, obviate such through the control of relative volumetric flow rates of the inner and outer layers in such a way as to shift the position of the core and control also the relative thickness of the inner and outer layers of the article.
Another object is to provide novel apparatus and methods to inject the leading edge of the interior core layer on the zero gradient of the combined velocity profile during the initial portion of the interior core layer flow, and then to change the relative volumetric flow rates of the inner and outer layers to cause the later or subsequent portion of the interior core flow to be offset from the zero gradient of the combined flow velocity profile.
An additional object is to provide novel apparatus and methods to restrict either the flow of the inner or of the outer layer volumetric flow rate in order to shift the interior core layer trailing portion inside or outside the 50% streamline through the nozzle and into the molded part.
Still a further object of the present invention is to provide a novel method and apparatus to produce an article of three materials molded into four layers.
Another object is to provide in such apparatus, novel operation wherein the leading edge of one of the interior core layers is injected on the zero gradient of the velocity profile of the combined stream before the start of flow of the other interior core layer.
Other and further objects will be described hereinafter and are more fully delineated in the appended claims.
In summary, however, from one of its important aspects, the invention embraces a method for co-extruding multiple plastic materials as for injecting through a gate region into a mold cavity to produce a molded article, that comprises, co-extrusively flowing streams of plastic materials with at least one interior stream that is to serve as an interior core of a resulting molded plastic article within inner and outer streams of plastic material that serve as covering wall plastic material layers for the core; forcing the flowing streams to flow along concentric annular flow paths within and along a longitudinally extending tubular extruder nozzle to the cavity gate region; adjusting the flow streams initially to cause the core stream to start to flow at a region of substantially zero gradient in the transverse flow velocity profile of the extrusion; thereupon varying the relative volumetric flow ratio of the inner and outer layer streams after the zero-gradient flow of the core layer has started in order to offset the core layer flow from the zero-gradient and to shift the core layer closer to one of the inner or outer annular flow boundaries, thereby to produce a molded article wherein the major portion of the core layer is closer to one of the inner or outer article walls than the other.
Preferred and best mode designs and configurations are later described in detail.